This invention relates to apparatus for driving a timing device and, more particularly, to such apparatus wherein the timing device is driven primarily from an external AC power source and, in the event that the AC power source fails, from an auxiliary DC source, such as a storage battery.
In many applications, a timing device, such as a clock or clock system, must maintain an accurate time indication during prolonged periods. For example, the clock system may comprise a master clock which is used to produce timing pulses for the control of plural slave clocks. This type of system is used in institutions and other facilities wherein it is desired to provide many time indicators, all of which are synchronized and all of which accurately indicate the correct time. As another example, the clock system may comprise but a single time indicator which necessarily should maintain and indicate accurate time.
In the foregoing, as well as other examples, it is advantageous to drive the timing device by electric motive power. Although this can be achieved by providing a local power supply together with various time generating and driving circuits, it is convenient to utilize the AC power which is commercially provided by utility companies. Virtually all of the major utility companies in the United States are members of a nationwide power network whose frequencies are maintained at 60Hz with a tolerance of .+-.0.2Hz on an instantaneous basis and a maximum accumulated error of .+-.0.25 cycles in 24 hours. This power network customarily holds any accumulated drift of the network frequency within these fixed limits by comparing the frequency with time signals from the National Bureau of Standards. Therefore, it is highly advantageous and economical to drive the timing device from commercially available utility power lines, provided that the timing device is synchronized with the power network frequency of 60Hz.
However, there always is the possibility of a failure in that part of the utility power system which is used to supply AC power to the timing device. In that event, not only is the source of driving energy interrupted but the timing information inherent in the 60Hz frequency also is interrupted. Consequently, the timing device which is powered from an external AC power source will be de-energized for the duration of the failure. It is desirable to account for this possibility of an AC power failure so as to continue to drive the timing device during such an interruption. Various proposals have been suggested to accomplish this alternative, or auxiliary, energization of the timing device. For example, a small inexpensive standby alternating current source can be provided to supply the necessary timing information and driving energy in the event that the external AC source fails. That is, a standby time base source can be used to supply driving energy and timing information to the timing device. Such a standby or auxiliary system is described in U.S. Pat. No. 3,643,420 which issued to Arthur W. Haydon on Feb. 22, 1972. The standby time base source generally may comprise a battery and suitable apparatus which is driven by the battery to supply the necessary timing information. Such circuitry may be constituted by a motor driven pulsing circuit or may comprise a conventional solid state oscillator. In any event, it has been thought heretofore that since long periods of time can transpire before an AC power source failure occurs, the relatively short "shelf-life" of commercially available batteries would require that the battery be of the rechargeable type. That is, since such batteries often could not maintain a sufficient charge to thus supply energy of a satisfactory level for long periods of time, it would be necessary to occasionally or periodically recharge the battery. This technique is described in U.S. Pat. No. 3,685,278 which issued to Arthur W. Haydon on Aug. 22, 1972.
Unfortunately, it has been found that the use of rechargeable batteries requires that a charge controlling circuit be provided. Also, the charging of a battery adds to the power requirements of the overall clock system. Accordingly, there has been a desire to avoid the use of rechargeable batteries, if possible.
Also, in the event of a failure of the AC power source, it is necessary that the local time base, or oscillator, provide the proper timing information substantially instantaneously. That is, the timing information provided by the local time base source must be synchronized with the AC power source frequency. Although synchronism generally can be properly maintained during normal operation by using the timing information derived from the AC power source as a synchronizing control signal, such synchronism cannot be readily maintained during the interruption of such timing information. Furthermore, many oscillating circuits which have been used heretofore as a time base source are not very stable over prolonged periods of time. That is, while the proper synchronism may be exhibited initially during a power failure, the local oscillator frequency and phase have a tendency to drift because of ambient temperature, wear, and the like, in the absence of the application of a synchronizing signal. Even the use of a calibrating circuit, such as is disclosed in U.S. Pat. No. 3,690,059 which issued to Arthur W. Haydon on Sept. 12, 1972, has not fully solved this problem of oscillator drift during periods of power interruptions.